Intermixing Of Layers Research Articles

Structure of Interfaces in Multilayers

ABSTRACTStructure of interfaces in as-deposited state and after annealling at elevated temperatures was studied in variety of dc-magnetron sputtered multilayers by X-ray diffraction and high resolution electron microscopy. Interfaces in multilayers from materials without phase equilibrium (Mo/Si, W/Si, MO/B4C, Cr/C, Mo/(B+C)) are unstable and has the trend to evolution due to interlayer redistribution of chemical elements. Formation of the extended intermixed transition zones at interfaces is characteristic for multilayers from materials without phase equilibrium with each other. Unhomogeneous structure of materials along and across layers is responsible for nonuni-form proceeding of layer intermixing, development of interface roughness and differences of neighbour interfaces. It was shown that thickness asymmetry of intermixed transition zones at Mo/Si and Si/Mo interfaces in large-period Mo/Si multilayers is caused by different mechanisms of growth of Mo and Si on each other and by different structure of lower and upper parts of polycrystalline Mo-layers. Interfaces in multilayers from materials with phase equilibrium MoSi2/Si, WSi2/Si, OB2/C, TiC/C, Ni/C, MO2B5/B4C) are sharp and stable at elevated temperatures which gives the possibility to treat them for modification of their structure (smoothening, restoring sharp profiles of chemical elements). Multilayers W/Ti and W/TiN showed trend to formation of epitaxial superlattices with coherent or partially coherent (with misfit dislocations) interfaces and pseudomorphic state of layers.

In-situ sputter deposition of PT and PZT films on Platinum and RuO 2 electrodes

The in-situ growth of PbZr xTi 1−xO 3 (PZT) and PbTiO 3 (PT) thin films by reactive sputter deposition has been investigated for applications with silicon substrates. In-situ deposition from three simultaneously operating magnetron sources was applied. At the selected growth temperature range (550–600 °C), self-stabilisation of lead composition by a large desorption rate of Pb on stoichiometric PZT surfaces has been observed. Interlayer mixing was found to be limited to a few monolayers. Both Pt and RuO 2 on SiSiO 2 wafers were used as top and bottom electrodes. The type of bottom electrode, together with the lead flux and the presence of a bottom PT layer to help nucleate the growth of PZT, was found to have a marked influence on the crystallisation behaviour of the PZT films. Dielectric characterisation has been performed on the films, and the dependence on the deposition parameters is discussed.

Improved thermal stability of AlGaAs–GaAs quantum well heterostructures using a ‘‘blocking’’ Zn diffusion to reduce column-III vacancies

Data are presented on the reduction of layer intermixing (disordering) in AlGaAs–GaAs quantum well heterostructures (QWH) during high-temperature anneals by an initial low-temperature ‘‘blocking’’ Zn diffusion. Room-temperature photoluminescence measurements of the increase in the lowest electron-to-heavy-hole transition energy in the QW are used to characterize the extent of layer intermixing. Doped (C and Si) samples annealed (850 °C, 12 h) after a low-temperature blocking Zn diffusion (480 °C) exhibit reductions in energy shift from ∼177 meV to as little as ∼18 meV. Similar effects are also observed, but to a lesser extent, for undoped samples. The improved thermal stability is attributed to a Zn-diffusion induced reduction in the number of column-III vacancies in the active layers and is confirmed by secondary-ion mass spectroscopy measurements.

Intermixing produced by ion implantation: Case of II–VI semiconductor multilayers

II–VI semiconductor multilayers, containing a quantum well (ZnTe/CdTe/ZnTe or Cd0.6Hg0.4Te/HgTe/CdTe) have been implanted with Zn+ or Cd+ ions at different fluences and temperatures, and analyzed by Rutherford backscattering spectroscopy and channeling. Direct backscattering of channeled ions (He+) experiments show that the density of crystalline defects in the well increases with the fluence and the temperature of implantation, and confirms that layer intermixing takes place. A model has been used to link the accumulation of defects to vacancy trapping in the region where the vacancy free enthalpy is minimized; it happens that in the cases under study this trapping corresponds to a decrease of the elastic energy of the strained layer. Intermixing rules have also been analyzed.

Numerical simulations of the Arabian Sea using tracers as proxies for phytoplankton biomass

A set of numerical simulations were conducted to determine the potential interannual variation of phytoplankton biomass in the Arabian Sea. A four‐layer, reduced gravity model was forced with interannually varying, monthly mean winds for the years 1981 to 1990 and a seasonally varying, monthly mean climatological heat flux. Passive tracers, with fixed source and decay rates, were used as proxies for phytoplankton. The tracers were released in the second layer, and upwelling combined with interlayer mixing transported the tracers into the first (the top) layer of the model. In the top layer advection, diffusion and specified decay rate produced distributions of tracer concentration that we compare with coastal zone color scanner (CZCS) images. In addition to maps of tracer distribution, we constructed time series from areal averages of tracer concentration. The time series are particularly useful for analyzing the temporal variability of the tracer concentrations. There was substantial interannual variability in the results and a strong seasonal signal associated with the southwest monsoon. The years of maximal tracer concentration were 1990 and 1985. The first was associated with a decadal maximum in wind speed and the second with a large wind stress curl over the western Arabian Sea. The years with minimal tracer concentration were 1981 through 1983. We compare our results with published analyses of composited CZCS images during the southwest monsoon in 1981 and 1982. We find general agreement between our results and the CZCS images during the peak of the southwest monsoon; however, we find that during the early phase of the monsoon, when the winds are strengthening rapidly with time, the CZCS images are badly aliased by year to year changes in the time periods used to composite available satellite data.

Potassium-induced reconstructive phase transitions on Au(100)

The adsorption of potassium on the quasi-hexagonally reconstructed (“5 × 20”) Au(100) surface has been studied in the temperature range between 130 K and 300 K employing LEED, AES, XPS, and work function (Δф) measurements. At room temperature, an intermediate work function maximum is correlated with a transition of the quasi-hexagonal substrate surface into a (1 × 2) phase caused by a missing-row reconstruction of the Au(100) surface. The lifting of the quasi-hexagonal Au reconstruction is also accompanied by a sudden decrease of the potassium 2p32 core level binding energy by ∼ 0.9 eV. Further adsorption of potassium leads to the formation of complex LEED patterns (among others a c(4 × 4) phase) until the monolayer saturation is attained around Θ ≈ 0.38. Lowering the surface temperature to 130 K does not stabilize the clean surface reconstruction sufficiently to allow formation of stable potassium overlayer phases on the (“5 × 20”)-reconstructed Au(100) surface; instead Δф and LEED measurements at 130 K indicate disorder and partial inhibition of the transition into the reconstructed (1 × 2) phase pointing to a thermally activated process. All in all, we confirm results of a recent STM/LEED/AES study on the Au(100)K system and provide further evidence for the idea whereafter elevated potassium coverages may favor interlayer mixing and/or growth of a potassium—gold alloy.

Integration process for photonic integrated circuits using plasma damage induced layer intermixing

A new quantum-well intermixing process in GaAs/AlGaAs structures, based on ion bombardment damage, has been developed. Bandgap tuned lasers and extended cavity lasers have been fabricated. Results show that the quality of the material is still high after intermixing. Losses as low as 18 dB cm-1 have been measured in the passive waveguides of the extended-cavity lasers.

Electrical Properties of (Zn, Mn) Containing Multilayer Metallizations to p-Type InGaAs/InP

ABSTRACTThe specific contact resistance, pc, of Au/Zn/Au, Ni/Zn/Ni/Au, Pd/Zn/Pt/Au and Pd/Mln/Sb/Pd/Au contacts to p-In0.47Ga0.53As/ InP has been measured as a function of layer thickness of Zn or Mn. All of the as-deposited contacts were ohmic, with pc = 1−2 × 10−5 Ω cm2. Increasing thickness of the Zn layer above 200 Å in the Au/Zn/Au contacts resulted in a minor decrease in pc while producing no change in the Ni/Zn/Ni/Au metallization. For the as-deposited Pd/Mn/Pd/Au contacts, the value of pc was independent of thickness of the Mn layer but differences in pc emerged at annealing temperatures of ≥ 250°. The analysis of these structures by RBS has shown an extensive intermixing of the metal layers at an annealing temperature of 450 °. In the Pd/Zn/Pt/Au contacts, the value of pc was reduced to a minimum value of 8 × 10−6 Ω cm2 by annealing at a temperature of 500 °. An examination of the Pd/Zn/Pt/Au configuration by RBS has shown that the Pt layer acted as a barrier for the indiffusion of the Au.

A search for surface alloy formation in faceting induced by monolayer metal films: Pd/W (111) and Ni/W (111)

A monolayer of Pd on a W (111) surface will cause faceting of the surface upon annealing to T > 700 K, with the formation of {211} oriented microfacets. In order to learn whether surface alloy formation or intermixing of surface layers occurs during faceting we have employed low energy ion scattering (LEIS) , low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) to investigate the Pd/W (111) system. We find that 3.0 Å Pd deposited at room temperature almost completely covers the substrate W. In the temperature range from room temperature to ∼ 1125 K (the onset of Pd desorption) there is no intermixing of Pd atoms with W atoms: Pd atoms stay on top of the planar surface as well as the faceted surface. In comparison with the Pd/W(111) system, we have also studied the Ni/W (111) system which does not exhibit faceting behavior. Within experimental uncertainty we find that there is ~ 10% intermixing of Ni atoms with W atoms in the topmost surface layer, after annealing at 1175 K.

Kinetics of solid phase interaction between Al and a-Si:H

The kinetics of solid phase interaction between Al and a-Si:H have been investigated. The experiment led to the observation of low-temperature crystallization as has been reported. The crystallization temperature was found to be 300–350 °C from diffraction studies. From the x-ray photoelectron spectroscopy study, electron transfer from Al to Si was observed in the intermixing layer in samples annealed at RT and 200 °C whereas there is no evidence of the electron transfer for 300 and 350 °C annealed samples. To explain these results, a comparison is made with the interaction in the Cr/a-Si:H system previously reported and the interdiffusion model is proposed.

Pd/Zn/Pd/Au ohmic contacts to p-type In0.47Ga0.53As/InP

The electrical properties of Pd/Zn/Pd/Au ohmic contacts to p-type In0.47Ga0.53As/InP have been investigated as a function of the ratio of the interfacial Pd and Zn layers and the annealing treatment. For as-deposited contacts, the presence of an increasing thickness of interfacial Zn and Pd to ∼300 Å in the metallization resulted in a reduction in specific contact resistance, ρc, to a low value of 1.2×10−5 Ω cm2. Annealing of all of the contact configurations except the Zn=0 and 20 Å structures produced a reduction in ρc to a minimum value of 7.5×10−6 Ω cm2 at 500 °C. A critical thickness of the Zn≥50 Å and Pd≥100 Å interfacial layers was required in order to produce a significant reduction in ρc during annealing. These results have been interpreted in terms of the reaction between Pd and In0.47Ga0.53As and an associated doping at the near surface region by Zn atoms. Annealing of the contacts at temperatures of ≥450 °C resulted in significant intermixing of the metal layers and the In0.47Ga0.53As as revealed by Rutherford backscattering spectrometry and Auger depth profiling.

Laser Cladding in Pre-Made Grooves

Post machining of treated components after laser cladding are necessary in most cases. Depending on the quality of the chosen material this job could be laborious and time consuming. By preparing grooves of a certain design on the work piece before laser cladding, the excess material to be removed can be minimized and post machining operations facilitated. The paper describes experimental works with laser cladding in pre-made grooves on cast iron specimens to investigate the influence of special groove design on the cladding results. The results are discussed in terms of macro surface roughness, efficient layer thickness, residual stress in the cladded layer and base material intermixing in the cladding. The results indicate that laser cladding in grooves is a way to reduce cladding material consumption and to increase the coating rate.

Polarity propagation in the InSb/ alpha -Sn/InSb heterostructure.

Films of InSb{111}A,B grown on \ensuremath{\alpha}-Sn(111)/InSb{111}A,B by molecular beam epitaxy behave as if the polarity of their surfaces could receive that of the initial InSb substrates. InSb on \ensuremath{\alpha}-Sn(111)/InSb(111)B alternates the surface polarity with \ensuremath{\alpha}-Sn thickness. Below 5 bilayers (BL) \ensuremath{\alpha}-Sn, the growing InSb exhibits the B type of surface composed of Sb and beyond 5 BL it changes toward the A type. This is due to Sb segregation from the initial InSb(111)B substrate to the \ensuremath{\alpha}-Sn surface. On the other hand, InSb on \ensuremath{\alpha}-Sn(111)/InSb(111)A always grows with the A-type surface via formation of the intermixing layers of (${\mathrm{In}}_{\mathit{x}}$${\mathrm{Sn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$)Sb, decreasing the value of x with increasing InSb thickness.

In situ investigation of amorphous silicon-silicon dioxide interfaces by infrared ellipsometry

An in situ investigation of the a-Si:H-a-SiO2 interfaces by infrared phase modulated ellipsometry is presented. 20–30-Å-thick intermixing layers are clearly revealed at the interface whatever the deposition sequence. In particular, the presence of SiO and (O)SiHn bonds in the first monolayers of the growth of the ‘‘top’’ a-Si:H is identified. Likewise a hydrogen accumulation, revealed by SiH vibrations, is observed during the early stage of the growth of a-SiO2 on top of a-Si:H. The nature of the a-Si:H-a-SiO2 and a-Si:H-a-SiNx interfaces is compared.

TEM Study of MOCVD Grown InSb/GaAs Heterostructures with and without TMIn Predeposited Layers

ABSTRACTHigh quality InSb epilayers were grown on GaAs substrates by metal organic chemical vapor deposition using a two-step growth procedure involving trimethal indium (TMIn) predeposition. From transmission electron microscopy studies, we found that an interdiffusion layer of thickness of 10 Å forms at the interface when the substrate is exposed to TMIn for approximately 6 secs prior to the growth of the InSb filns. Hall mobilities up to σ 52,000 cm2/V-s were obtained at 300 K on a 2.1-μm-thick InSb heteroepitaxial film. In contrast, samples without TMIn predeposition showed polycrystallinity of the InSb films grown on single crystalline GaAs substrates. The effect. of TMNIn predeposition is to minimize the misorientation of the grains, suppress the polycrystallinity, decrease the density of threading dislocations, and increase the electron mobilities in the films. However, we found that too much TMIn predeposition gives rise t.o an intermixing layer at the InSb/GaAs interface which deteriorates the film quality. Details of the effect of the TMIn predeposition on the microstructure of InSb/GaAs with different predeposition times (zero, 6, and 12 secs) are discussed.

The Reliability of Multilevel Metallization on InGaAs/GaAs Layers

ABSTRACTNon-alloyed ohmic contacts using Ti/Pt/Au and Ni/Ge/Au on InGaAs/GaAs layers grown by Molecular Beam Epitaxy (MBE) have been investigated. The n-type InGaAs film has a doping concentration higher than 1X1019 cm-3. Specific contact resistance below 2X10-7 Ωcm2 could be easily achieved with Ti/Pt/Au. Due to the layer intermixing and outdiffusion of In and Ga, the specific contact resistance and sheet resistance increase after thermal treatment. When Ni/Ge/Au is used as the contact metal, the outdiffusion of In and Ga atoms is more severe than that of Ti/Pt/Au. After annealing at 450°C for two minutes, the Au4In formed and the characteristics of the contact became worse. All the phenomena illustrated above have been observed and investigated by Transmission Line Model, X-ray diffraction, Auger Electron Spectroscopy and Secondary Ion Mass Spectrum. As far as the thermal stability is concerned, it is convinced that Ti/Pt/Au is the best one of these two non-alloyed ohmic contact studied.

In situ investigation of amorphous silicon-silicon dioxide interfaces by infrared ellipsometry

An in situ investigation of the a-Si:H - a-SiO2 interfaces by Infrared Phase Modulated Ellipsometry (IRPME) is presented. 10–30 Å thick intermixing layers at the interface are clearly revealed whatever the deposition sequence. In particular, the presence of SiO bonds in the first monolayers of the growth of the “top” a-Si:H is identified. These IR results are confirmed by UV-visible ellipsometry measurements. The nature of a-Si:H - a-SiO2 and a-Si:H - a-SiNx interfaces are compared.

Shallow angle lapping of III-V semiconductor thin layer structures by an ion beam/chemical etching technique

Layer thicknesses of <2 nm have been resolved in GaAs-AlGaAs superlattice structures using a novel method involving the chemical etching of the shallow lapped area surrounding an ion beam sputter crater formed after Auger electron spectroscopy depth profiling. This method can be used to measure layer thickness variations across a wafer of a nominal 15 nm InGaAs layer in a pseudomorphic high electron mobility transistor. It can also be used to probe interlayer lattice mixing at selected areas on the wafer, for example, near an ohmic contact.

Raman scattering of α-Sn/Ge superlattices on Ge (001)

A Raman scattering study on short-period single-crystal α-Sn/Ge superlattices grown by low-temperature molecular beam epitaxy pseudomorphically on Ge (001) substrate is presented herein. The phonon spectra exhibit distinct superlattice effects, such as narrow first, second, and third order folded-acoustic modes showing a doublet splitting, which is evidence for the proper periodic compositional modulation of the structure. A peak at 260 cm−1 is attributed to a Sn–Ge vibrational mode. The Raman cross section resonance of this mode is redshifted compared to that of the Ge optical mode. There are also Ge bulklike contributions from two-phonon scattering and disorder activated modes present in the spectra. Annealing experiments show that, despite the negligible solid solubility of Sn and Ge, layer intermixing occurs up to complete alloying, while the crystal quality is preserved.

Ohmic Contacts to p-Type InGaAs/InP with a Graded Bandgap Heterobarrier

ABSTRACTOhmic contacts to p-type InP with an In0.47Ga0.53As buffer layer and an interposed superlattice of 50 Å In0.47Ga0.53As/ 50 Å InP have been investigated. Initial studies of contacts to In0.47Ga0.53As/ InP without the superlattice structure have shown that Pd/Zn/Pd/Au metallization produced a lower specific contact resistance (pc = 1.1 × 10−4 Ω cm2) than Pd/Ge/Au, and over a wider range of anneal temperature than Au/Zn/Au. The incorporation of the superlattice in the p-In0.47Ga0.53As/ InP structure resulted in Pd/Zn/Pd/Au contacts with pc of 3.2 × 10−5 Ω cm2 as-deposited and 7.5 × 10−6 Ω.cm2 after a 500 °C anneal. The presence of Pd/Zn in the metallization was shown as important in reducing pc. Significant intermixing of the metal layers and In0.47Ga0.53As occured at ≥ 350 °C, as revealed by Rutherford backscattering spectrometry.